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Bump Map

The Bump/Nor Map

A Bump or Nor map (in Blender terminology) is basically a grayscale map which adds detailed bumps and grooves to an object. In general, whiter areas represent bumps, and darker areas represent grooves. The closer to pure white, the higher the bump; the closer to pur black, the deeper the divot.

How do we create an effective bump map? Think about the wood. It has deep grooves due to deterioration from the salt air, but the bumps will be mild and worn from use. What's this mean? Recall how the bump map works: more white leads to higher, more black leads to lower. Therefore, the bump map should look somewhat darker overall. Not many whiter areas, but good, deep, black areas. To achieve this effect with the pineCol.jpg file, open up your favorite painting program. Since I cannot afford Photoshop yet, I'll be using the Gimp. Fortunatley, the methods described here will apply to both programs. If you don't have either program, you can still use your favorite one as long as it can change levels or brightness and contrast and create grayscale images.

Creating the Bump Map with the Gimp

Fire up the Gimp and open pineCol.jpg. Change the image to grayscale by right-clicking, selecting image -» mode -» grayscale. As you can see, the image is mostly light gray with a few black streaks through it. To change that, you'll use the levels editor, available in the menu as image -» colors -» levels. Here's how the levels editor works (please note: what I say here might be false by definition, but this is how I perceive it to work, and so far, it has. So nyah to you technical purists!):

The input levels will determine individual strength. That is, by moving the far left triangle right, the black parts become blacker; the far right triangle will make white parts whiter. The middle triangle will determine which triangle (left or right) will be favored (i.e. have a greater effect). The image needs more contrast; so bring the far right and left triangles closer. Favor the darker regions, to get more groove than bump. My settings are shown in figure 4.

My particular example was too black. The output triangles will change the image, making it overall more black or more white. The left triangle decreases the amount of total blackness, and the right decreases the total amount of whiteness. Adjust accordingly and save the picture as pineNor.jpg. Remember, if you don't have a levels editor, you can achieve similar effects by tweaking brightness/contrast settings.

From the Gimp to Blender

To try this texture bump map on the wooden plank, add the texture into the second texture channel of the wood material with default settings. In the materials buttons, make sure you change the output mapping from "col" to "nor." Because Blender handles Nor maps differently, press the nor button a second time (making it yellow) to inverse. Render the scene to see the results. If your version isn't bumpy enough, there are at least two solutions: go back to the picture editor and increase the contrast for the map, or simply increase the bump effect within blender. I increased the \emph{nor slider} from the default of 0.5 to 1.0.
Figure 4: Example Levels Settings
Example Levels
Figure 5: Nor Map Settings
Nor Map Settings
Figure 6: Applied Nor Map
Applied Nor Map
Rendering will produce the ribbed plank as in Figure 6. For the sake of realism, it still misses a major element of detail: internal shading. You really cannot tell how deep those grooves are at first glance. When you have a groove in real life, it becomes darker the deeper it gets. This is a result of the diffusion of light, and appropriately so, diffusion maps can be used to simulate this natural effect.
   

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